The present invention relates to an A-C current limiting device (hereinafter simply referred to as current limiting device) for limiting transient defective current that flow when a line is grounded or is short-circuited in an electric power system such as A-C lines or A-C transmission lines, or among different electric power systems.
A conventional device of this type proposed so far is as shown in FIG. 1, in which a current limiting device 1 is inserted for one phase between coupling points a, b of two systems S.sub.1 and S.sub.2. A reactor 11 and a capacitor 12 are connected in series to form a first circuit, and a reactor 11A and resistors 14, 14A are connected in series to form a second circuit. The first and second circuits are connected in parallel. A closing means 13 consisting of a gap, contact elements, semiconductor elements or non-linear elements, is inserted between a connection point d of reactor 11 and the capacitor 12 of the first circuit, and a connection point c of the resistors 14 and 14A of the second circuit. The closing means 13 is in an open state under normal conditions and only conducts when an applied voltage thereacross exceeds a predetermined voltage.
The thus constructed device of FIG. 1 operates as described herebelow.
Under ordinary condition, the systems S.sub.1 and S.sub.2 have nearly the same potential, and the potential difference is small across the connection points c and d. Therefore, the closing device 13 remains open, and a series resonance circuit is formed by the reactor 11 and the capacitor 12 to establish resonance operation for the A-C frequency in the system. Under this condition, the systems S.sub.1, S.sub.2 have impedances close to zero and are coupled together through the first circuit.
On the other hand, in case an accident has developed, a heavy transient current flows through the systems S.sub.1, S.sub.2 and the potential difference increases suddenly across the connection points c and d. The closing device 13 therefore works to short-circuit the connection points c and d, whereby a parallel resonance circuit is formed by the capacitor 12 of the first circuit and the reactor 11A of the second circuit. Namely, the systems S.sub.1, S.sub.2 are coupled together through a large impedance (particularly, the impedance increases infinitely if the resistance of resistor 14A is zero), and an excess current flowing between the systems S.sub.1 and S.sub.2 is limited. If the resistor 14A has a resistance which is zero, an infinitely large impedance is produced when the reactor 11A and the capacitor 12 are connected in parallel, and a great current limiting effect is provided. After the current is interrupted between the systems S.sub.1 and S.sub.2, the inductive energy stored in the reactor 11A and the capacitive energy stored in the capacitor 12 continue to flow as a circuiting vibration current through a closed circuit b-c-d which consists of reactor 11A, resistor 14A, closing device (CLD) 13 and capacitor 12. To convert this energy into heat energy and to radiate it, therefore, the resistor 14A must have a suitable resistance. When the resistor 14 has a zero resistance, furthermore, the inductive energy stored in the reactor 11 generates a surge voltage outside the current limiting device 1, for example, on the circuit breaker of the side of the system S.sub.1 or S.sub.2, and causes the circuit breaker to be subjected to stringent breaking duty. In order to convert the energy into heat energy and to radiate it in the current limiting device 1, therefore, the resistor 14 must also have a suitable resistance.
The current limiting device 1 which has been proposed thus far and which is shown in FIG. 1, is constructed and operates as mentioned above. This device, however, has the following defects:
(1) Two reactors are separately provided which makes the setup complex; and
(2) Two resistors are separately provided which further makes the setup complex.